1. Field of the Invention
The present invention relates to a catalyst for cleaning exhaust gas discharged from an internal combustion engine and, more particularly, to an exhaust gas cleaning catalyst that can be promptly activated immediately after the start of the engine.
2. Description of the Related Art
Various types of catalysts have been proposed for cleaning and removing components such as nitrogen oxides, carbon monoxide, hydrocarbons, and the like, contained in exhaust gas discharged from an internal combustion engine such as an automobile engine.
A representative type among these exhaust gas cleaning catalysts is an exhaust gas cleaning catalyst consisting of a carrier substrate such as a honeycomb substrate of cordierite, having a multiplicity of cells, and which has, on the surface of the exhaust gas channels thereof, a coating of a catalyst carrier layer selected from metal oxides such as alumina, zirconia, ceria and the like, with metal selected from platinum (Pt), palladium (Pd), rhodium (Rh), and the like, carried by the catalyst carrier layer (see, for example, Japanese Patent Publication No. 2001-205109).
However, in order to make full use of the function of such an exhaust gas cleaning catalyst, it is necessary to raise temperature of the catalyst above its activation temperature. Thus, in an atmosphere at a low temperature such as that found immediately after engine start, the catalyst is not always in an activated state and exhaust gas cleaning capability of the catalyst cannot be fully utilized. There is, therefore, a problem that the capability of a catalyst can be fully utilized only when the temperature of the catalyst is raised above a certain activation temperature.